It's not 'required' or 'forced', it's just strongly recommended. Chances are this shoddy vendor was close to not getting ceritifcation, and needed some brownie points.

And the reason why is that it makes XP run much better. Boot times, making Shut Down work properly, etc are all improved by ACPI. These are good things for customers.

Running OSs that don't support ACPI is not a major concern when trying to set guidelines for hardware that works best for XP. I don't see why Windows XP certification has to take into account the hardware support of other OS's? They are XP guidelines.

Is there a reason that ACPI can't be supported by a Free OS? Fill me in on the limitation.